The present invention relates to the transfer of collected particulate matter. More specifically, this invention relates to a novel device and method for redeposition of particulate matter from one type of medium to another type of medium which is more suitable for use in connection with microscopic examination and analysis of individual particles.
Specimens of particulate matter present in air, water, biological systems and other sources are routinely collected for analysis. These specimens have been generally characterized by bulk chemical analysis which provides an average composition of a sample, but completely ignores information concerning individual particles within the heterogeniety of the sample.
Individual particle analyses have relied on sophisticated microscopic techniques, such as analytical light microscopy (ALM), scanning electron microscopy (SEM), laser microprobe mass analysis (LAMMA) and scanning transmission electron microscopy (STEM). However, such techniques have been employed only qualitatively for individual particle analysis. But with the recent advent of computer control of analytical processes coupled with image analysis individual particle characterization techniques can now be placed on a quantitative basis. That is, statistically significant populations of particles can be characterized over shorter time periods, e.g. 500-1000 features/hour. Accordingly, sample preparation for these types of analyses becomes more critical in developing accurate information and data.
Typically, samples of particulate matter are collected on glass fiber and cellulose filter medium. In the case of a sparsely loaded sample, particles can become intimately associated with the collection media's fiber matrix and are often deposited deep within the media. In the case of heavier loading of particles on collection media, there is a tendency of particle overlap occurring, and consequently, a lower resolution of individual particle analysis. In each of these techniques, individual particles should be redeposited onto a smooth, flat, chemically simple surface, e.g. polycarbonate membrane, and dispersed in such a manner that the individual particles of the specimen are analyzable.
Heretofore, particle transfer and redeposition mechanisms relied on three basic techniques: physical transfer by "wetted" needles; "vacuum cleaning" from the original filter and backflushing by solvents while treated with ultrasonic energy. Such methods have not been totally satisfactory for reasons that physical transfer removes only surface particles from the original membrane providing neither qualitative nor quantitative transfer. Vacuum cleaning methods, although more reliable than physical transfer, nevertheless are not quantitative.
Typically, backflushing in combination with ultrasonic agitation has involved the steps of initially placing a filter sample on aluminum foil backed with paperboard whereupon multiple disks are cut from the original sample for extraction of the particulates. Each disk is placed between perforated stainless steel screens, installed in a pressure filtration holder and immersed in a container of solvent, e.g. acetone, containing the probe tip of a sonic dismembrator. Filtered acetone is forced through the assembly and the direction of flow reversed. The solvent and particulate mixture are collected together in a separator container. The flushing would be repeated and reversed several times for each filter section. The mixture would then be conducted through a more suitable filter for individual particle analysis. As can be readily observed, this system fails to provide control of the area subjected to the redeposition process. That is, besides increasing the risk of physical contamination of the sample and loss of particles through multiple process steps, such methods employing the backflush approach failed to provide means whereby small known areas of original filter could be flushed so that the particles are redeposited on a known area of "new" substrate. Thus, the loading mass per unit area which is critical to efficient processing of individual particle analysis results was not appropriately taken into account. Too heavy a loading can impede accurate analysis of individual particulates due to particle overlap, whereas too light a loading detracts from time-efficient analysis. Accordingly, there is a need for an improved device and method for redepositing individual particles in preparation of samples for analysis, particularly on a micro scale.
The present invention provides a novel redeposition spot sampler device and method for substantially complete and potentially quantitative release and transfer of particulate matter from a collection medium to a more suitable medium for microscopy and individual particle analysis. That is, the immediate invention provides the means to quantitatively control in a uniform and homogeneous fashion the desired level of increase or decrease in particulate loading per unit area of "new" filter media, or in other words, the density or closeness of a population of reseated particles on a more favorable media for more accurate analysis. Although the device can be any size, it is especially well suited for micro scale analysis of individual particles permitting the use of only a small portion of the original medium while allowing preservation of the balance of the medium for other analytical procedures, e.g. bulk analysis.
In addition, this invention also provides convenient alternative means for redepositing particles relating, for instance, to fractional redeposition of a sample, either chemically or physically.